Fix clang-tidy issues - Analyze usage of endl to eliminate unnecessary buffer flushing - performance-avoid-endl

[eljonny]

I use std::endl a lot and it might not be necessary everywhere I'm using it.
In general, in this library in particular, since the test code being run is very likely to cause a crash, I think in most cases it's reasonable to use std::endl, but again this might not be the case in every instance.
I need to go through on a case-by-case basis and suppress performance-avoid-endl where appropriate, and fix (probably change to \n) where appropriate.